In electrophotographic copying machines involving image development with colloidal dispersions of pigment in a low conductivity liquid carrier, one is likely to obtain copies which are deficient in quality at reasonable machine speeds if the original to be copied contains printed areas which are of the order of about 3/16 inch or greater in width unless a so-called "development electrode" is employed. The development electrode is usually a metallic conductive device, sometimes grounded, other times "floating" electrically, and sometimes biased with an appropriate potential, placed near or in contact with a photoconductor surface bearing a charged image during the development step. The development electrode serves to improve the ability of the developer or "toner" to fill in the interior parts of wider regions of the copy by locally modifying the strength of the electrostatic field of the image area as the field extends into the toner. In the absence of an electrode, the field strength is greatest at the edges of an electrostatic image and falls off rapidly toward the interior of the image. The strength of the field determines how many oppositely charged toner pigment particles will migrate to the charged image; therefore, the image becomes developed to a high density at the edge and to a lower density at the center. In the case of extremely large black areas on an original, a relatively low density print develops at the center of the image on the copy while a relatively high density print develops at the edge of the same image on the copy. This results in a copy which gives a poor appearance. When a development electrode is employed, the field strength is modified by it so that the strength becomes more nearly uniform across a large image area. The developed image of the large area also becomes more uniform in density and results in a copy with an improved appearance.
In "conventional" liquid toners, many of the colloidal pigment particles are present as either flocculates or as agglomerates. Flocculates are clumps of particles which are dispersed at some point in time but which, either because of insufficient like particle charge or because of opposite particle charge, have since come together and grown in size. Agglomerates, in contrast to flocculates, are groups of particles which were never completely dispersed in the toner carrier liquid.
When a conventional liquid toner is employed in conjunction with a development electrode, and particularly with an electrode which contacts the photoconductor surface, there is observed a drop in image density of fine line copy and an increase in the "background" or non-image density. It is believed that this occurs because the high field strength causes rapid motion of the charged toner particles in the liquid. Since many of the toner particles are loosely-held-together flocculates or agglomerates, they break apart, some of their fragments having considerably reduced charge and possibly even charge of opposite sign to the bulk of the particles in the toner. Increased background density can result from a localized greater number concentration of particles of smaller size resulting from the breakup of flocculates or agglomerates and/or from the sudden localized presence of oppositely charged fractions in the toner. It obviously would be beneficial to be able to utilize a development electrode without the attending increased background difficulty.